The invention relates to railway signaling relays and particularly to relays used in railway signalling apparatus and particularly for broken rail detection systems. Such relays are widely used and a model which has particularly wide use is known as the "B" relay. Forms of that relay are described in greater detail in U.S. Pat. No. 4,564,829, and U.S. Pat. No. 2,258,122 both assigned to the same assignee as the present application. The relay description in these patents is incorporated herein by reference.
Although the invention also has particular application to bias neutral relays, it will be understood that the invention also has application to conventional neutral relays. Bias neutral relays have been around for a long time and are characterized by a magnet disposed near the pole pieces that works on the armature. The relay includes a magnetic shunt that is near the coil of the relay. The relay is "biased" in the sense that you can effectively only energize or pick the relay with the current in a single direction. In the opposite direction the current required to pick the relay may be as much as 40 times the current required in the opposite direction of current flow. A neutral relay can be picked or energized with either polarity direct current.
A broken rail detection system typically has the two rails of an axial section of track electrically isolated from the end abutting sections of track by the installation of insulating joints in each rail at two axially spaced locations. A battery is coupled to the rails at one axial location and the coil of a relay is connected to the two rails approximately five or more miles away. Any break in either rail causes the relay contacts to separate and cause a warning signal to be displayed to the engineer operating a locomotive on that section of track. The broken rail detection circuit is short circuited by the wheels of a train and thus the relay may also be released due to this effect.
For many years, circuits of this type were designed specifically for train detection and secondarily to provide broken rail detection. Currently, many railroads are adding fail safe radio detection equipment and want to remove the train detection track circuits while retaining the broken rail detection capability. It is thus desirable to optimize the circuit for just detecting broken rails even though such a system cannot consistently detect the presence of trains on a given section of track. In other words, such a system may or may not detect the presence of a train on a track system. By optimizing the system for broken rail detection, the circuit can be three to four times as long as what would be required if both reliable train and reliable broken rail detection were essential. The track circuit for just detecting broken rails can be as much as ten miles in length as opposed to a typical track circuit for detecting trains which seldom can be more than about three miles in length. Therefore, if all that is desired is to detect broken rails and not detect trains, only about 1/3 as much apparatus is required to cover the same length of track or the same amount of equipment or cover about three times as much track. Thus, the cost of the system is substantially reduced. The maximum length of track being protected is obtained if the resistances of the circuits at both the battery end of the rails and the relay end of the rails are made as low as possible.
It is common practice to position alternating current transmission lines parallel to railroad tracks because the easement for the railroad tracks often allows power lines to be located along the same right of way. Power lines may also be positioned along side the railroad tracks to power electrically powered trains as well as signalling circuits. Particularly, in foreign countries alternating current power is used to power the trains and thus those power transmission lines are particularly likely to induce alternating current currents in the rails. These lines can all contribute to inducing alternating current voltages in the tracks and this can interfere with satisfactory operation of a broken rail detection system. More specifically, induced alternating current in the rails can cause a relay to be energized at voltage and current conditions that will cause a broken rail detection system to give a false assurance of a safe operating condition. Thus, a malfunction of the broken rail detection system may create a serious risk of personal and property injury.
Traditional approaches used in the United States to avoid a problem with the induced alternating current are (1) to make the line circuits very short which means you have to use a lot more relays and equipment and/or (2) to bury a heavily shielded power transmission cable in the ground. The latter approach involves spending a lot of money to bury a special cable which could be avoided by merely using an alternating current immune relay.
The prior art includes the apparatus shown in FIGS. 1 and 2. These relays have been made in Great Britain. The apparatus of FIG. 1 includes a special airgap structure which provides an alternating current path so that very little of the alternating current flux flows through the armature and thus does not affect the operation of the relay to provide alternating current immunity. In this structure, a core C, a waist W and a pole extension PX directing alternating current to the yoke Y. A small amount of the alternating current flows through the pole P to the armature A and then to the yoke Y.
FIG. 2 illustrates a structure with a relatively heavy monolithic magnetic shunts which directs both the alternating current and the direct current. Because of the heavy copper slugs CS a high reluctance is provided to the alternating current so that essentially just the direct current flux flows through the armature A. The copper slugs CS act like an inductor or shorted turn that also provides an impedance to the alternating current flux flow. Because the copper slugs CS act as a shorted turns, some relays having this feature may unfortunately have a much higher alternating current flow through the coils C, C. The known copper washer slugs used in the prior art have been used strictly to provide longer operating times. In other words, to make the relay a slow pick up relay or a slow drop away relay.
The prior art includes relays made and sold by the assignee of the present invention which have utilized a plurality of copper slug washers for the purpose of varying the timing characteristics of the relay. Magnetic shunt lamina have not been used in the prior art to the best of the applicant's knowledge.
It is an object of the invention to provide a method for and a relay apparatus for providing a substantial immunity to induced alternating current voltages and which will thus promote safe railroad operation.
It is another object of the invention to provide an AC immune relay that does not increase the resistance of the operating coil and thus render the relay less suitable for use in a broken rail detection system.
It is another object of the invention to provide apparatus which will have adjustable alternating current immunity, adjustable sensitivity and adjustable timing characteristics and further that such adjustability can be readily varied to make a relay meet unique specifications for particular applications and which will minimize the requirements to maintain inventory.
It is yet another object of the invention to provide apparatus which will utilize substantially all of the component parts of existing production relays and thus will be inexpensive to manufacture.
It is another object of the invention to provide a relay which can have substantially the same pick up (energization) and drop away (de-energization) voltage and current characteristics as the existing relay which has not been modified.
It is an object of the invention to provide an alternating current immunized relay utilizing a magnetic structure means which does not insert additional resistance in the relay circuit and does not require external components for the relay.
It is a further object of the invention to provide apparatus which, at least in some embodiments, will have less sensitivity than conventional relays and thus will be less vulnerable to noise pulses.
A further object of the invention is to provide a more efficient shunt path for alternating current than has been provided by monolithic magnetic shunts in the past.